The present invention relates to a washing method for cleaning workpieces such as semiconductor wafers and the like, and to a washing apparatus for us in such washing method.
With advancements in the integration of semiconductor devices in recent years, wiring of circuits has become finer and wiring distances have become smaller. If dust greater in size than a wiring distance exists on a semiconductor substrate of such highly integrated semiconductor devices, short circuiting of wiring occurs. Accordingly, the allowable size of dust remaining on semiconductor wafers must be suppressed to be smaller than a wiring distance.
As a method for washing semiconductor wafers and the like, there have so far been disclosed a number of methods including a so-called scrubbing washing method or a method of scrubbing a surface of a semiconductor wafer with a nylon or mohair brush or the like or with a sponge of PVA (polyvinyl alcohol); an ultrasonic washing method or a method of jetting water endowed with ultrasonic vibration energy onto the surface of wafer and thereby cleaning off dust; a method of jetting cavitation-producing high-pressure water against a wafer to wash the wafer therewith, and a combination of these methods.
Now, in an ultrasonic washing method or a washing method using cavitation-producing high-pressure water, in order to jet water to the entire surface of a wafer, pure water is jetted toward a wafer from a nozzle a jetting hole of which is oriented to the surface of the wafer while the wafer is horizontally held and rotated at a predetermined number of revolutions and while the nozzle is moved with a constant velocity to traverse over the wafer so that the locus of the moving jetting hole and thus the locus of the point of collision of jetted water with the surface of the wafer passes the center of rotation of the wafer.
When pure water is jetted against a wafer under high pressure or at high velocity, static electricity arises where the pure water collides with the wafer. Especially according to the above-mentioned ultrasonic washing method or the washing method using cavitation-producing high-pressure water of the prior art, the center of rotation of the wafer is located under the locus of the shifting jetting hole for a longer period of time than the other portions of the wafer, because the apparatus is so constructed that pure water is jetted from a nozzle and the locus of the jetting hole of the nozzle passes the center of rotation of the wafer.
Therefore, when high-pressure pure water is jetted against the wafer or high-velocity pure water is jetted against the wafer by the ultrasonic washing method, a central portion of the wafer is exposed to collision with high-pressure or high-velocity pure water over a longer period of time than the other portions of the wafer. As a result, static electricity arises at a central portion of the wafer, thereby causing electrostatic breakage of electronic circuits at a surface part of the central portion of the wafer. Thus, short circuiting occurs and a defective product may be formed.
As a method of preventing the occurrence of static electricity mentioned above, there has been proposed a method of injecting carbon dioxide gas into ultra-pure water and thereby lowering a specific resistance of the ultra-pure water.
However, this method is extremely uneconomical and the apparatus therefor is complicated because the carbon dioxide originally present in the raw water must be first removed by passing the water through a decarbonater and an anion exchange resin tower with carbon dioxide thereafter being added again. In other words, there has so far been no effective method for preventing the occurrence of static electricity at the central portion of a wafer.